Go: Pointer to type is incompatible with interface

Question

I'm not sure how to formulate the question properly. I have 2 go code snippets that should do the exact same thing but apparently they don't, since one works and the other doesn't (doesn't compile)

func writeSomething(writer *io.Writer) {
}

func main() {
    file, _ := os.Create("error.log")
    var logWriter io.Writer = file
    writeSomething(&logWriter)

}

func main2() {
    file, _ := os.Create("error.log")
    writeSomething(&file)

}

Playground

main() works and main2() doesn't.

prog.go:20:17: cannot use &file (type **os.File) as type *io.Writer in argument to writeSomething: *io.Writer is pointer to interface, not interface

The only difference is that I used an intermediate variable but I'm not doing any pointer referencing or dereferencing.

What am I doing wrong here?

Error message is exact. Just remove indirection in function signature func writeSomething(writer io.Writer) you need not it any case — Uvelichitel
Ok thanks, that works. But I would still be interested why the first version works even though it does the sam thing — Robi
In main you convert file to interface io.Writer and then pass writeSomething a pointer to the interface which is just what writeSomething expects. — md2perpe
In main2 you have a pointer to os.File and then pass a pointer to that pointer to writeSomething. Go cannot convert that to a pointer to interface io.Writer. — md2perpe

Pavlo Strokov Pavlo Strokov · Accepted Answer · 2018-02-18T11:59:46

You shouldn't use pointers to interfaces, it is a bad practice in Go.
And about code that doesn't compile - when you create an intermediate variable logWriter you allocate an interface variable, that has its own allocated memory to describe what it stores (actual type) and a reference to the actual structure.
That is why &logWriter and &file have different types.
But as I sad previously, you should not use pointers to interfaces, just use interface type and do not get address of interface type variable.

Go: Pointer to type is incompatible with interface

1 Answers